Many types of communications between locations rely on the use of the public switched telephone network (PSTN). The most typical use of the telephone network is the dial-up of a directory number to establish a talking path between the originating station and a destination station, whereupon the telephone line remains busy for as long as the audio or data signals are carried. After the communication is terminated, both stations go on-hook and the telephone line is idled.
There exist other systems, some of which are utilized in mobile radio communications, in which the public switched telephone network is utilized to provide both data and audio communications between plural cell switching systems and a centralized hub controller. In this type of environment, the communications are of the dispatch type, or better known as push-to-talk (PTT). Each time a mobile radio is keyed, an idle radio channel is selected and the originating party may typically speak for several seconds, in which event the audio signals are transmitted via the mobile radio antenna to a local cell switching system. The cell switching system then transmits the dispatch radio signals to other mobile radios in the radio transmission area, and also transmits the audio signals via a telephone line through the PSTN to the hub controller. Based on the group to which the originating mobile radio is assigned, the hub controller bridges the telephone line to the corresponding telephone lines of yet other cell switching systems so that all the mobile radios assigned to the group can receive the audio signals. In this manner, the other cell switching systems that are located outside the radio transmission range of the transmitting cell can nevertheless receive the audio portion of the dispatch communications. Even though the communications between the various mobile radios may only be a few seconds each, the telephone lines interconnecting the cell switches to the hub controller are always "off-hook", thereby remaining ready to carry any of the dispatch audio signals. It can be appreciated that if a telephone line had to be dialed and established for each communication, the delay period would be unacceptable and the bidirectional communication of audio signals would be extremely cumbersome and burdensome.
Many mobile radio systems are configured so that one data line, which is typically a telephone line carrying modem signals, is coupled between each cell switch and the hub controller. Associated with each data line are one or more telephone lines for carrying the audio dispatch communications. Depending upon the configuration of the system, the telephone lines can be idled, if not used within a period of several minutes. In such a system, during periods of low usage, at least one audio line will always remain active, and the remaining audio lines may be automatically disabled, but reactivated should the dispatch traffic increase to a level such that one audio line cannot handle the traffic conditions without encountering significant delays.
While the public switched telephone network is generally a reliable medium of carrying either audio or data signals, service interruptions are known to occur. The reliability of telephone switching systems is generally a function of the quality of the systems, the age and type of technology employed, as well as the maintenance of the equipment and lines and often the weather or environmental conditions in which the equipment and lines are required to operate. The reliability of telephone switching systems is vastly different in different countries. For example, in the United States and many other highly developed and industrialized countries, the telephone switching systems are very reliable and taken for granted by the public. In many other countries, the telephone switching systems are themselves old and constructed with old technology, are poorly maintained, and thus interruptions in service are frequent.
Telephone communications can be interrupted for various reasons including inadvertent on-hook conditions by the telephone systems, substantial line static or cross-talk, insufficient loop current, audio cut-off, inadequate or lack of line signaling, circuit failures in the line cards, erratic cross-connections in the switching fabric, physical telephone line crosses or breakage and a host of other problems. It can be appreciated that if a telephone line is immediately required to carry audio or data signals, a service disruption is not only annoying, but can cause substantial delays in the reestablishment of the communication path. Also, if data is being transmitted over the telephone line, then an interruption in service will often require a retransmission of the entire string of data. While one solution to the problem of inadequate communication service is the overall improvement of the public-switched telephone network, such a solution is often too expensive to consider. Also, while maintenance and routine testing can serve to improve the quality of communication services, this solution may also be inadequate and expensive for old and unreliable systems.
From the foregoing, it can be seen that a need exists for a technique to minimize the interruption in service between telephone line users of a public-switched telephone network. Another need exists for methods and apparatus for maintaining a telephone line in an off-hook status, especially if some of the lines are prone to failure. Yet another need exists for a rather inexpensive method of testing the telephone lines to assure an operational status thereof between the periods of usage. Yet another need exists for a line test that is carried out at predefined times to minimize interruption of service should the line be defective.